Time Division Multiple Access (TDMA) systems have the ability to selectively send signaling from a base repeater to a transmitting subscriber to enable features such as power control and transmitter disable. The mechanism to signal the transmitting subscriber is via the air interface protocol. The air interface protocol divides a slot into 100 into three main components as illustrated in FIG. 1. The first component 102 carries control information common to both channels on an outbound path called the common announcement channel (CACH). The second component 104 carries the voice or data payload in the slot 100. The third component 106 carries signaling information (e.g., synchronization and embedded signaling) for the receiving subscribers.
The ability for subscribers to receive while transmitting may be handled by one of three ways in a TDMA system. First, the subscriber radio may radio frequency (RF) duplex meaning that it may transmit and receive on different frequencies simultaneously. This adds substantial cost to the subscriber. Secondly, the subscriber may use time division duplex with two different clocks to allow for very fast switching between a transmit and receive frequency. This too adds the cost of a second frequency generator to the subscriber. Finally, a subscriber may be produced with only one frequency generation unit and still be allowed to switch between frequencies in a time division method but a slower rate than that afforded by a subscriber with two frequency generation units. It is this lowest cost method with which the present invention is concerned.
The channel that sends the signaling to a subscriber while it is transmitting is called the reverse channel. It can readily be seen that the reverse channel signaling must be located in the shared signaling field of the slot. This means that the reverse channel signaling must share bandwidth with synchronization and other signaling (e.g., link control) for the receivers as illustrated in FIG. 2. For example, the other signaling may comprise an indication of the current talkgroup or identification of the transmitting party.
Thus, there exists a need to share bandwidth of the shared signaling field between the reverse channel and other signaling for the receivers, while minimizing impact on audio/data delay and minimizing resource loading on the subscriber.